doc/man3/OPENSSL_malloc: clarify OPENSSL_clear_free() w/ NULL

doc/man3/OPENSSL_malloc: clarify OPENSSL_clear_free() w/ NULL

It wasn't explicitly clear that it was safe to call OPENSSL_clear_free()
with a NULL because, as worded, it sounded like it may call
OPENSSL_cleanse() on NULL before calling OPENSSL_free().

CLA: trivial

Signed-off-by: dann frazier <dann.frazier@chainguard.dev>
Reviewed-by: Saša Nedvědický <sashan@openssl.org>
Reviewed-by: Eugene Syromiatnikov <esyr@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Neil Horman <nhorman@openssl.org>
MergeDate: Thu Mar 12 17:43:51 2026
(Merged from https://github.com/openssl/openssl/pull/28127)

(cherry picked from commit 4adb40393c073fd5ac34495efd083e981a6a6d69)